MCAT Biology and Biochemistry: 6 High-Yield Practice Questions
The Biological and Biochemical Foundations section of the MCAT (BBFL) tests molecular biology, biochemistry, and physiology in the context of passages and standalone questions. These 6 questions cover the highest-tested patterns.
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update Updated October 2026
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Built by surgery and anesthesiology residents·Written for the MCAT
High-Yield Topics Covered
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Enzyme kinetics (Km, Vmax, inhibition types)
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DNA replication and repair
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Protein structure and folding
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Glycolysis and citric acid cycle
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Cell signaling pathways
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Membrane transport
Question 1 of 12Score: 0 / 0
Q1Enzyme Kinetics
An enzyme has a Km of 2 mM and Vmax of 100 μmol/min. A competitive inhibitor is added at constant concentration. How do Km and Vmax change?
Explanation: Competitive inhibition: the inhibitor competes with substrate for the active site and can be overcome by increasing substrate concentration. Effect: apparent Km INCREASES (enzyme appears to have lower affinity for substrate), Vmax is UNCHANGED (sufficient substrate can still achieve maximum velocity). Noncompetitive inhibitor: Km unchanged, Vmax decreases.
Q2DNA Replication
During DNA replication, the enzyme responsible for synthesizing a new DNA strand from 3' to 5' on the template is:
Explanation: DNA polymerase reads the template strand 3'→5' and synthesizes the new strand 5'→3'. This means the enzyme travels along the template in the 3'→5' direction to add nucleotides in the 5'→3' direction. Primase makes the RNA primer; helicase unwinds the helix; ligase seals Okazaki fragments.
Q3Protein Structure and Enzyme Regulation
A protein is described as having four subunits, each with its own active site. Binding of substrate to one subunit increases substrate affinity at the other subunits. This describes:
Explanation: Positive cooperativity: binding of substrate to one subunit of a multimeric enzyme causes conformational change that increases substrate affinity of remaining subunits. Classic example: hemoglobin binding oxygen (though hemoglobin is not an enzyme). Sigmoidal (not hyperbolic) kinetics on substrate-velocity curve.
Q4Cell Signaling
A cell signaling pathway involves a ligand that binds a receptor, activates adenylyl cyclase, increases cAMP, and activates PKA. This is a:
Explanation: Gs-coupled GPCR → activates adenylyl cyclase → increases cAMP → activates PKA (protein kinase A). Examples: epinephrine (β-adrenergic), glucagon, PTH. Gi-coupled GPCRs inhibit adenylyl cyclase. RTK pathways activate RAS/MAP kinase. JAK-STAT is used by cytokines. SMAD is TGF-β.
Q5Metabolism and Bioenergetics
During high-intensity anaerobic exercise, pyruvate is converted to lactate. The primary purpose of this reaction is:
Explanation: Lactate dehydrogenase converts pyruvate to lactate while oxidizing NADH → NAD+. The regenerated NAD+ is essential for continued glycolysis (which requires NAD+ as an electron acceptor in the glyceraldehyde-3-phosphate dehydrogenase step). Without this recycling, glycolysis halts. Lactate is later converted back to glucose (Cori cycle) in the liver.
Q6Membrane Transport and Physiology
A mutation in the Na+/K+ ATPase causes it to become permanently inactive in nerve cells. The immediate consequence would be:
Explanation: The Na+/K+ ATPase maintains resting membrane potential by pumping 3 Na+ out and 2 K+ in. If inactivated, Na+ accumulates intracellularly and K+ is lost → membrane depolarizes toward 0 mV. The neuron loses the ability to maintain a resting potential and cannot generate normal action potentials.
Q7Genetics
Two heterozygous parents (Aa × Aa) have a child. What is the probability the child shows the recessive phenotype?
Explanation: A Punnett square gives AA, Aa, Aa and aa. Only aa (1 of 4) shows the recessive phenotype, so the probability is 1/4.
Q8Physiology
Which hormone increases water reabsorption in the collecting duct?
Explanation: ADH (vasopressin) inserts aquaporins into collecting-duct cells, increasing water reabsorption. Aldosterone mainly increases sodium reabsorption and potassium secretion.
Q9Metabolism
Where does the citric acid (Krebs) cycle take place in eukaryotic cells?
Explanation: The citric acid cycle runs in the mitochondrial matrix. Glycolysis is cytosolic, and the electron transport chain is in the inner mitochondrial membrane.
Q10Molecular biology
Which enzyme removes RNA primers during DNA replication in prokaryotes?
Explanation: DNA polymerase I removes RNA primers (5′→3′ exonuclease activity) and fills the gaps with DNA. Ligase then seals the nicks; primase makes the primers; helicase unwinds the helix.
Q11Cell biology
Which organelle modifies, sorts and packages proteins for secretion?
Explanation: The Golgi apparatus modifies proteins (for example, glycosylation), sorts them and packages them into vesicles. Proteins destined for secretion are first made on the rough ER.
Q12Biochemistry
At a pH above an amino acid’s isoelectric point (pI), what is its net charge?
Explanation: When pH is above the pI, the amino acid has lost more protons than at its pI, so its net charge is negative. Below the pI it is net positive.
The Biological and Biochemical Foundations (BBFL) section is 59 questions and comprises about 25% of the MCAT composite score. Within BBFL, approximately 25% is biochemistry, 65% is biology, and 10% is general chemistry. The MCAT emphasizes biochemistry heavily — enzyme kinetics, amino acid properties, metabolism, and molecular biology are all routinely tested.
What is the difference between Km and Vmax?
Km (Michaelis constant) = substrate concentration at which reaction velocity = ½ Vmax. It is an inverse measure of enzyme affinity — lower Km = higher affinity (enzyme reaches half-max velocity at low substrate). Vmax = maximum reaction velocity when enzyme is saturated with substrate. Competitive inhibitors increase apparent Km (lower affinity) without changing Vmax. Noncompetitive inhibitors decrease Vmax without changing Km.
What metabolism pathways are highest yield for MCAT?
Highest-yield metabolic pathways: glycolysis (10 steps, ATP yield, key enzymes), pyruvate dehydrogenase complex (acetyl-CoA production), citric acid cycle (NADH/FADH2 production, key intermediates), oxidative phosphorylation (ATP synthase, electron transport chain), and the Cori cycle (lactate → liver → glucose). Also high-yield: fatty acid oxidation (beta-oxidation), ketogenesis, and gluconeogenesis (key irreversible steps).
Are these questions like the real MCAT?
They practice the same concepts, but most real Bio/Biochem questions are passage-based. Use AAMC’s official practice materials to get used to passages, and use these to check core concepts quickly.
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